1. Field of the Invention
The invention relates to a DC—DC converter of the voltage reduction-buck type with the PWM (pulse width modulation) method in which the efficiency is increased, and a device for operating a high pressure discharge lamp such as a metal halide lamp, mercury lamp or the like, using a DC—DC converter.
2. Description of the Related Art
Conventionally, of the converters which convert the voltage of a DC source into another value, output it and supply it to a load, i.e., DC—DC converters, the DC—DC converter of the voltage reduction-buck type, which is shown in FIG. 18 is often used to carry out voltage reduction-conversion.
In this circuit, the current from the DC source (Vin) is repeatedly shifted by a main switching device (Qx′) such as a FET or the like into the ON state or the OFF state, and a smoothing capacitor (Cx′) is charged via the main coil (Lx′). In this arrangement, this voltage can be applied to a load (Zx).
During the interval in which the above described main switching device (Qx′) is in the ON state, charging of the smoothing capacitor (Cx′) and current supply to the load (Zx) are carried out directly by the current through the main switching device (Qx′), and moreover, in the main coil (Lx′), energy is stored in the form of a flux. During the interval in which the main switching device (Qx′) is in the OFF state, the smoothing capacitor (Cx′) is charged via a fly-wheel diode (Dx′) by the energy stored in the form of a flux in the main coil (Lx′) and current is supplied to the load (Zx).
This converter is operated under PWM control of the main switching device (Qx′). Specifically, by feedback control of the ratio between the time interval in the ON state and the sum of the period of the ON state and the period of the OFF state of the main switching device (Qx′), i.e., the continuity ratio, the voltage supplied to the load (Zx) can be adjusted, even as the voltage of the DC source (Vin) fluctuates, to a desired (for example, constant) value, the supplied current can be adjusted to a desired value and the supplied wattage can be adjusted to the desired value.
Of course, the value of the desired efficiency (voltage, current, wattage or the like) can assume a constant value or it can also be changed over time. For feedback control of the desired efficiency, a detector is needed to determine the output voltage and the output current, as is a feedback control circuit, which is not shown in the drawings.
FIG. 16 shows the voltage and current waveform of this converter using one example. If the main switching device (Qx′) is shifted into the ON state, the voltage (VxD′) supplied to the main switching device (Qx′) passes from the voltage of the DC source (Vin) essentially to 0 V. However, this transition does not take place instantaneously, but requires a certain time.
Here, in the process in which the voltage (VxD′) of the main switching device (Qx′) gradually decreases, the current (IQx′) of the main switching device (Qx′) also gradually begins to flow. There is therefore an interval during which neither the voltage (VxD′) nor the current (IQx′) is 0. According to the size of the time integral of the product of the voltage and the current, for each transition of the main switching device (Qx′) into the ON state a switching loss (SwL) occurs on the main switching device (Qx′).
This switching loss also arises by the same process in the case of the transition into the ON state as in a transition into the OFF state. However, normally, the loss in the transition into the ON state is greater. The reason is that when the main switching device (Qx′) is a FET, for example, a parasitic electrostatic capacitance is present between the source electrode and the drain, that the electrical charge which has been charged onto this electrostatic capacitance during the interval of the OFF state of the main switching device (Qx′) at the voltage of the DC source (Vin), in the transition into the ON state is subjected to forced short circuit discharge, and that the energy which is consumed in doing so is added to the switching loss (SwL).
When this switching loss is present, there is not only the disadvantage of a reduction in the efficiency of the converter, but also the disadvantage of a large converter and a cost increase of it, since the heat generation of the main switching device (Qx′) is large and since therefore a switching device with large maximum power dissipation must be used and furthermore there must be a large radiator with high radiation efficiency in addition. Furthermore, the fan that supplies cooling air for cooling the radiator must be a high capacity fan, which brings the disadvantages of the reduction in the efficiency and the increase in size and cost of the converter.
In order to eliminate these disadvantages, conventionally, a host of proposals have been made. They are mainly technologies that prevent intervals during which neither the voltage (VxD′) nor the current (IQx′) is 0. Normally, the technology in which switching is carried out at a 0 voltage of the switching device, is called zero voltage switching, and the technology in which switching is carried out at a 0 current of the switching device, is called zero current switching. Often, using a so-called LC resonance the voltage applied to the switching device and the current flowing in the switching device are temporarily taken over by the voltage induced by the L component (coil) and the current flowing in the C component (capacitor) and are essentially set or reduced to 0, and during this time a transition of the switching device into the ON state or the OFF state is carried out.
For example, in Japanese patent document HEI 1-218352, a DC—DC converter of the voltage reduction-buck type with current resonance is proposed. In this proposal, the current flowing in the main switching device (Qx′), however, due to resonance has a higher peak value than a conventional DC—DC converter of the voltage reduction-buck type. Therefore, it becomes necessary to use a switching device with a high current. Furthermore, in the case in which the switching frequency is higher than the resonant frequency, it is possible that the loss continues to increase because the switching device is shifted into the OFF state at a high current.
Additionally, in this circuit arrangement, according to the assumption of a constant output voltage for a DC—DC converter, the PWM method is undertaken with a constant switching frequency. Because of this, it is necessary to match the continuity ratio thereof to the resonant frequency. The range of the continuity ratio is therefore limited. An increase of the efficiency can therefore only be accomplished in the vicinity of the rated output voltage. Neither a guideline nor conditions for a measure against the fluctuation of the load were considered.
Furthermore, for example, U.S. Pat. No. 5,880,940 discloses a DC—DC converter of the voltage reduction-buck type in which a secondary winding is added to the main coil (Lx′), and thus, a transformer is formed.
In this proposal, a DC—DC converter is described as being operated by connecting an auxiliary switching device to the transformer as a forward converter. However, an increase of the ripple in the output current by this operation was not even considered. The added auxiliary switching device cannot be subjected to zero voltage switching either. It is necessary to add another coil and to carry out zero current switching.
In the case of zero current switching, different from zero voltage switching, there is specifically the disadvantage that the problem of power consumption loss as a result of the forced short circuit discharge is not eliminated in the transition of the electrical charge into an ON state which was charged in the parasitic electrostatic capacitance of the main switching device. Therefore, this is not ideal.
On the other hand, if the use of a DC—DC converter of the voltage reduction-buck type is considered, the resonant conditions of the LC resonance circuit are easily satisfied in a stable manner, since the output voltage is relatively stable for applications such as a constant voltage current source or the like.
In the case of use as a device for operating a high pressure discharge lamp such as a metal halide lamp, a mercury lamp or the like, however, the lamp voltage as the output voltage is changed significantly by the state of the lamp as a load. Under certain circumstances it fluctuates steeply. Therefore, a specially adapted construction is needed. The converter must also be matched to this construction.
The feature of the high pressure discharge lamp as the load of the converter is described below. Generally, a high pressure discharge lamp (Ld) has an arrangement in which a discharge space (Sd) is filled with a discharge medium which contains mercury and in which a pair of opposed electrodes (E1, E2) is located for the main discharge. Between the electrodes (E1, E2), an arc discharge is produced and the radiation emitted from the arc plasmas is used as the light source.
The high pressure discharge lamp (Ld), in contrast to a general load, exhibits a property which is closer to a Zener diode than to an impedance element. This means that the lamp voltage does not change greatly, even if the flowing current changes. A lamp voltage which corresponds to a Zener voltage however changes greatly depending on the discharge state.
Specifically, in the state before the start of the discharge, the Zener voltage is extremely high because no current at all is flowing. If, by operating a starter, such as a high voltage pulse generator or the like, a discharge is started, a glow discharge is formed. In the case, for example, of a discharge lamp which contains greater at least 0.15 mg of mercury per cubic millimeter of volume of the discharge space (Sd), the glow discharge voltage ranges from 180 V to 250 V. In the state before the start of the discharge, a voltage of at least to the glow discharge voltage is applied to the high pressure discharge lamp. Normally, this voltage is roughly 270 V to 350 V and is called the no-load voltage. The starter is operated in this way.
When the electrodes (E1, E2) are heated by the glow discharge to a sufficient degree, a sudden transition into an arc discharge takes place. Immediately after the transition a low arc discharge voltage from 8 V to 15 V is shown. This is a transient arc discharge. The arc discharge vaporizes the mercury and if heating of the mercury vapor continues, the arc discharge voltage gradually increases until it reaches a steady-state arc discharge from 50 V to 150 V. The voltage in a steady-state arc discharge, i.e., the lamp voltage, depends on the density of the mercury which has been added to the discharge space (Sd) and the distance between the electrodes (E1, E2).
Immediately after the transition into the arc discharge, depending on the vapor state of the mercury, the glow discharge suddenly returns or the arc discharge and the glow discharge takes place alternately in a vigorous back and forth manner.
At a constant voltage from the DC source (Vin), the output voltage of the DC—DC converter of the voltage reduction-buck type is at a value which is obtained by multiplying roughly the voltage of the DC source (Vin) by the continuity ratio. Therefore, the DC—DC converter of the voltage reduction-buck type can be kept approximately for the DC-constant voltage current source.
On the other hand, in idealized switching theory in the case in which a DC-constant voltage current source is connected to a Zener diode as a load, i.e., still another DC-constant voltage current source, the theory fails and good analysis is not possible. More accurately, when in the case of connecting a Zener diode as the load to a constant voltage current source, the output voltage of the constant voltage current source is lower than the Zener voltage, no current at all flows in the Zener diode. Conversely, in the case in which the output voltage of the constant voltage current source is higher than the Zener voltage, an infinitely large current flows.
When a discharge lamp which can be roughly regarded as a Zener diode is connected to a realistically present DC—DC converter of the voltage reduction-buck type as a load, extinction of the discharge occurs in the case in which the output voltage of the converter is lower than the Zener voltage. Conversely, in the case in which the output voltage of the converter is higher than the Zener voltage, an unduly high current which is determined by the current serviceability of the DC source (Vin) and of the converter flows in the lamp.
Therefore, in a device for operating a high pressure discharge lamp, the following is required of a converter for supplying a high pressure discharge lamp:
There is a demand for the property which enables a prompt change of the continuity ratio in a wide, variable range for PWM control according to the discharge voltage of the high pressure discharge lamp in order to prevent extinction of the discharge from occurring or an unduly large current from flowing and the lamp and converter circuit from being damaged. These must be achieved even at a discharge voltage that corresponds to the no-load voltage which changes in this way to a great extent and also vigorously depends on the discharge state, i.e., the state in which a no-load voltage is applied (state before the start of discharge), the glow discharge state, the state of a transient arc discharge, or the steady-state arc discharge state. Furthermore, there is a demand for a property that enables maintenance of operation in which the switching loss is reduced by resonant operation.
In the case of high ripple which is contained in the current flowing in the discharge lamp, there is a case in which instability, flicker and extinction of the discharge arise due to acoustic resonance. Therefore, it is required of the converter that the ripple of the output current is small. Accordingly, it is necessary to prevent the operation of the resonant circuit which is arranged for reducing switching loss from accelerating the formation of a superfluous ripple component.
In the case, for example, of a DC—DC converter of the voltage reduction-buck type which is described in the above cited U.S. Pat. No. 5,880,940, the main coil also acts as a transformer with a resonant oscillation effect. Originally, during the interval in which the main switching device is in the ON state, in base operation of the DC—DC converter of the voltage reduction-buck type, on its two ends, the main coil has a voltage difference between the supplied DC source voltage and the output voltage and works in such a way that the input DC source voltage is not applied directly to the load.
In the case of a great fluctuation of the output voltage, of course, the voltage on the primary side of the transformer fluctuates greatly with a resonant oscillation effect. Since the energy transmitted to the secondary circuit of the transformer also fluctuates greatly because of the resonant oscillation effect, as a result the resonant operation also fluctuates greatly. The DC—DC converter of the voltage reduction-buck type described in U.S. Pat. No. 5,880,940 is therefore not suited as a converter for supplying a high pressure discharge lamp.
As was mentioned above, it is necessary in a DC—DC converter of the voltage reduction-buck type to reduce the switching loss in order to avoid raising the size and costs of the converter. However, in the prior art, it was difficult to have a wide, variable range of output voltage and keep down the cost because of the addition of the resonant circuit. In particular, it was difficult to obtain a converter that is suited to operate a high pressure discharge lamp.